In recent years there is a growing need for printers able to print faster and at higher print densities. Laser printers, which employ a laser light source, and LED (light-emitting diode) printers, which employ an LED array as the light source, are two examples of printers used in response to such needs. While a laser printer requires the use of a mechanical mechanism, such as a rotating polygonal mirror, for the scanning laser beam, an LED printer only requires that the light-emitting diodes (hereinafter also referred to as "light-emitting elements") of the light-emitting diode array be electrically controlled. LED printers do not have any mechanical moving parts and can thus be made smaller, faster and more reliable than laser printers.
FIG. 1 is a cross-sectional view of a conventional (prior art) LED array 10 used as a light source of an LED printer. LED array 10 comprises a substrate 14 of n-type conductivity GaAs having a pair of opposed surfaces 16 and 18. A layer 20 of n-type conductivity GaAsP about 80 microns thick is on the surface 16 of the substrate 14. Spaced apart diffused regions 22 of p-type conductivity are in the layer 20. The diffused regions 22 are about 1.5 microns deep. A masking layer 24 of SiN.sub.x is on the layer 20 and has openings 26 therethrough exposing the diffused regions 22. A separate conductive electrode 28 is on each of the diffused region 22, and a conductive electrode 30 is on the surface 18 of the substrate 14. An anti-reflection layer 32 of SiN.sub.x is over the masking layer 24, electrodes 28, and the exposed surfaces of the diffused regions 22. Each of the diffused regions 22 forms with the layer 20 a separate light-emitting element 12. Only two light-emitting elements 12 are shown. A p-n junction 34 between each of the diffused region 22 and the layer 20 is an emission region.
The LED array 10 is made by first depositing the GaAsP layer 20 on the surface 16 of the substrate 14 using vapor phase epitaxy (VPE). The masking layer 24 is then deposited on the layer 20 and is defined using standard photolithographic techniques and etching, to form the openings 26. The diffused regions 22 are then formed by diffusing a p-type conductivity dopant, such as zinc, into the layer 20 through the openings 26 in the masking layer 24. The electrodes 28 and electrode 30 are then formed by deposition and alloying. Finally, the anti-reflection layer 32 is deposited.
For application as a light source of a printer, the qualities generally required of an LED array are a high emission efficiency and uniform intensity of the emitted light. In the case of the conventional (prior art) LED array 10, the GaAsP layer 20 has a high internal absorption and there are high-density lattice defects caused by a lack of lattice matching between the GaAsP of the layer 20 and the n-type GaAs of the substrate 14. Also, the material itself has low emission efficiency and considerable non-uniformity. Moreover, the light-emitting region is a p-n homojunction, which from the standpoint of emission efficiency, is not the most suitable type to use for light source applications. A further problem is that of blurred printing caused by the fact that the intensity of the emitted light within the element declines in proportion to the increase in the distance from the electrodes.
Referring to FIG. 2, there is shown a cross-sectional view of a conventional (prior art) AlGaAs single heterojunction type light-emitting diode array 36 which overcomes the drawbacks of the conventional GaAsP light-emitting diode array 10. The diode array 36 comprises a substrate 38 of p-type conductivity GaAs having a pair of opposed surfaces 40 and 42. A first layer 44 of p-type conductivity Al.sub.x Ga.sub.1-x As is on the surface 40 of the substrate 38. The first layer 44 is about 10 microns thick and is doped with zinc to a concentration of 5.times.10.sup.17 impurities/cm.sup.3. A second layer 46 of n-type conductivity Al.sub.y Ga.sub.1-y As is on the first layer 44. The second layer 46 is about 5 microns thick and is doped with Te to a concentration of 8.times.10.sup.17 impurities/cm.sup.3. For emitting light with a wavelength of 720 nm, the aluminum concentration in the first and second layers 44 and 46 is x=0.2 and y=0.5 respectively. Spaced grooves extend through the second layer 46 and a portion of the first layer 44 to form mesas 48, each of which forms a light-emitting element.
A third layer 50 of n+type conductivity GaAs is on a portion of the second layer 46 of each of the mesas 48. The third layer 50 is about 0.1 microns thick and is doped with Sn to a concentration of 5.times.10.sup.18 impurities/cm.sup.3. A separate electrode 52 is on the third layer 50 of each mesa 48, and an electrode 54 is on the surface 42 of the substrate 38. An anti-reflection coating 56 is over each of the mesas 48 and the portion of the first layer 44 between the mesas 48.
The array 36 is made by first depositing the first layer 44 on the surface 40 of the substrate 38 using liquid-phase epitaxy (LPE). This is followed by the deposition of the second layer 46 on the first layer 44, and the material of the third layer 50 on the second layer 46. The material of the electrode 52 is then deposited on the third layer 50 and the electrode 54 is deposited on the surface 42 of the substrate 38.
The electrodes 52 are then defined using photolithography and plasma etching. Using the electrodes 52 as a mask, the exposed portions of the material of the third layer 50 are then removed to form the third layer 50 under each of the electrodes 52. The third layer 50 is etched with NH.sub.4 OH:H.sub.2 O.sub.2 =1:10.
H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O=1:2:40 chemical etching is then used to form the grooves through the second layer 46 and about one micron into the first layer 44. This forms the mesas 48 which are the light-emitting elements. The orientation of the substrate is taken into account to ensure that the shape of each mesa 48 is such that the portion adjacent the electrode 52 is in the reverse mesa direction, i.e., the sides of the mesa taper inwardly in the direction away form the electrode 52. The portion of the mesa 48 away from the electrode 52 is in a forward mesa direction. The fabrication process is then completed by using plasma CVD (chemical vapor deposition) to form an anti-reflection SiN.sub.x coating 56, followed by alloying to form ohmic contacts between the electrodes 52 and 54 and the third layer 50 and substrate 38 respectively.
Thus, structurally, this is an array of discrete high-luminance LEDs. This arrangement suppresses optical energy attenuation caused by internal absorption by using the Al.sub.y Ga.sub.1-y As second layer 46, which is transparent to light emitted by the Al.sub.x Ga.sub.1-x As first layer 44. Also, as well as using a junction with good epitaxial matching characteristics, the heterojunction improves the carrier injection efficiency and results in an overall external output efficiency that is several times higher than that of the light-emitting diode array 10 shown in FIG. 1.
However, unlike the coherent light of a laser, LEDs emit light in all directions. One problem with the array 36 is that light emitted toward the substrate 38 is absorbed. This results in a corresponding loss of external output. Light travelling horizontally and impinging on the sloping surface of the mesas 48 is lost by being scattered or absorbed by this sloping surface. Thus, the only light that is emitted to the exterior is the light directed toward the LED surface. This limits the external output efficiency to values of no more than several percent.